This invention relates to a novel position sensor in which a reactive position transducer is incorporated in a resonant circuit tuned to the drive frequency of the sensor. More particularly, it relates to a sensor driven by a square wave, in which the resonant circuit is tuned to the fundamental frequency of the square wave and thus provides a high degree of attenuation at harmonics of that frequency.
A typical application of sensors to which the invention is directed is a galvanometer unit comprising a servo-controlled, limited-rotation motor. An angular position sensor connected to a shaft of the motor provides a feedback signal for a servo loop that controls the instantaneous angular position of the shaft. The transducer is a variable reactance device, such as a compasitive transducer arranged in a differential capacitor configuration. The relative values of a pair of capacitances are linearly related to the angular position of the shaft, whereas the total capacitance is essentially independent of the shaft position.
The galvanometer units are used in devices such as optical scanners, laser cutting tools and the like and it has become increasingly desirable to reduce the sizes of these devices. In turn this has imposed a requirement that the sizes of the galvanometer units be decreased, with a resulting requirement that the position sensors be miniaturized. However, this reduces the capacitances in the sensor and the sensitivity of a capacitance transducer is roughly proportional to these capacitances. The decrease in capacitance can be offset to some extent by increasing the frequency. However, it is desirable to obtain a sensitivity that is greater than what can be obtained solely by a frequency increase. The invention is therefore directed to a reduction in the size of the sensor without a corresponding reduction in its sensitivity.
A sensor incorporating the invention includes a square-wave generator and a series-resonant circuit comprising a capacitive position transducer and an inductor. The sensor is a differential capacitor and its total capacitance is essentially independent of changes in the sensed position. The capacitance and inductance resonate at the fundamental frequency of the generator and therefore serve as a filter that removes the harmonics of that frequency. Furthermore, in a series resonant circuit the voltage across each of the reactive elements is much greater than the voltage across the circuit. Accordingly, the voltage across the capacitor is much greater than the drive voltage. This eliminates the requirement for a transformer that would otherwise be used to obtain an acceptable sensitivity when the size of the transducer is reduced.
Equivalently, the sensitivity of the sensor increases with the current through the transducer. The low impedance provided by series resonance results in a much larger current for a given drive voltage and the transducer can therefore be driven with the voltages common to integrated circuits. The circuitry can therefore be found on a semiconductor chip, which maintaining the desired sensitivity.